1. Field of the Invention
This invention pertains generally to drying liquid and solid particulate materials, and more particularly to an apparatus and method for efficiently drying products of various viscosity and sizes.
2. Description of the Background Art
Devices and methods for evaporation of liquids and drying of solid material are used in chemical, pharmaceutical, food processing, and other industries. Frequently, drying devices use balls, pellets, or other generally spherical contact bodies as drying or heating media to provide large drying surface areas. In using such devices and methods, a solution, suspension or dispersion of material is typically spread over a bed of plastic or metal balls within a drying chamber, and heat is applied, either by thermal conduction or application of heated air. The moisture evaporates, leaving solid material coating the balls. Agitation of the balls removes the dried solids. New liquid solution or suspension can then be applied, and the process repeated. Other drying devices heat the balls externally, then immerse the heated balls into the liquid to be evaporated. Generally, the balls are circulated or recycled in the drying chamber by a screw conveyor, bucket elevator, or the like.
Several devices and methods which use balls and pellets for drying and evaporation purposes have been disclosed. For example, U.S. Pat. No. 2,275,117 discloses a process and apparatus for preheating or drying slurry in which a rotary kiln has radially located compartments containing charges of generally round movable bodies. Slurry is introduced to the kiln and distributed over the movable bodies by rotating the kiln. Heated air is blown across the moving bodies, and the dried material passes through a perforated wall out of the kiln.
U.S. Pat. No. 3,442,769 discloses a method and apparatus for heating and distilling saline water wherein heated pebbles are added to saline water in a distillation vessel and vapor distilled therefrom, with the pebbles passing through an opening in the bottom of the vessel to a ball mill. From the mill, the pebbles are carried up by a screw conveyor or bucket elevator to a heater, and then added again to the distillation vessel.
U.S. Pat. No. 3,459,638 discloses a method for desalination of sea water using heated metal balls to vaporize water. Balls are conveyed through a furnace and fed into a still containing sea water. The balls are removed from the still and conveyed back through the furnace, and then the still again. Water vapor is distilled, and concentrated brine is removed from the bottom of the still.
U.S. Pat. No. 3,843,462 discloses a hot ball desalination process in which brine solution is introduced countercurrently to heated pellets. Water vapor is flash distilled from the brine, and the pellets are then reheated and new brine introduced.
U.S. Pat. No. 3,653,424 discloses a method and apparatus for concentrations solutions or suspensions or for recovering the dry substance thereof, in which liquid is spread onto a bed of interengaging spheres within a chamber, and air is forced through the spheres to effect drying. The spheres are continuously transported from the bottom to the top of the chamber by a screw conveyor. Dried solids escape through a grate at the bottom of the chamber.
U.S. Pat. No. 3,898,745 discloses a drying apparatus for concentrating solutions wherein liquid is applied to a bed of contact bodies in a chamber. The contact bodies are continuously circulated from the bottom to the top of the chamber by a screw conveyor, while air is passed through the bodies to achieve drying. Dried solids fall through a perforated base in the bottom of the chamber.
French Patent No. 608,014 discloses a process and apparatus for transmission of heat and cold wherein balls in an upper chamber are heated or cooled by a gas or liquid stream of desired temperature, then gravity fed to a lower chamber wherein the balls are used to heat or cool the contents of the lower chamber, for distillation or other purposes. The balls escape from the bottom of the second chamber and are conveyed by a bucket elevator to the upper chamber again.
As can be seen therefore, a variety of drying apparatuses and processes using balls or pellets are known. However, the currently used drying apparatuses suffer significant drawbacks. For example, as balls leave the screw conveyor or other ball circulating means, the balls tend to mound up in one area, resulting in an uneven bed of balls, thus reducing drying efficiency. This problem of mounding is especially prevalent in drying of sticky or tacky materials such as food products. Additionally, the existing art does not effectively allow even application of liquid or other materials to be dried onto the bed of balls, resulting in uneven distribution on the drying bed and reduced drying efficiency Further, many materials that are dried on an industrial scale are air sensitive and can suffer undesirable oxidation upon drying. Particularly, food products such as apples, and other fruits and vegetables discolor or lose flavor and fragrance when exposed to heated air. Use of an inert gas such as nitrogen in the drying apparatus helps avoid this problem, but available drying apparatuses are not well suited for use with inert atmospheres.
Therefore, there is a need for an apparatus and method for drying materials which provides for even distribution of balls when forming the drying bed, which provides for even application of materials to be dried over the drying bed, and which is suitable for use with inert or non-oxidizing atmospheres. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in prior drying devices and methods.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.